Heavy rains in various parts of the country this winter have caused soil to end up in locations where it otherwise would not be. These mudslides cause damage ranging from minor landscape repair to major structural work where mountainous soil has settled along the wall of a home.
Proper watershed, drainage systems and ground cover are a few ways to keep soil from eroding or moving. Where significant elevation changes in grade or a slope or hillside exist, a retaining wall may be needed.
Such a wall can be constructed of stone, brick, concrete, steel and wood and in varying configurations to correspond with its purpose. A small retaining wall (12 inches or less) can be constructed with minimal support. On the other hand, a retaining wall 3 feet high or greater will require support to keep it, and the earth that it is retaining, from moving.
As a matter of fact, many municipal building departments require a building permit for the construction of a retaining wall higher than 36 inches, without regard to the materials being used. Also, many building officials require engineering calculations along with the permit application. Whatever the height may be, it is imperative that the wall be designed and built to last for years.
One of the popular types of retaining walls of a height of 4 feet or less is wood timbers. Timber walls have a life span of 15 to 20 years if properly constructed. What's more, unlike stone or concrete, a timber wall can be constructed by a do-it-yourselfer with professional results. Due to the size of the timbers, the job requires a strong back and often a helper.
The key to a sturdy, long-lasting wall is the quality of the timbers. Pressure-treated material will last longer because the wood fibers are impregnated with pesticides such as copper chrome arsenate (CCA) to resist the rot that results from earth-to-wood contact. CCA is approved by the Environmental Protection Agency for use as a wood preservative. The use of old railroad ties should be avoided because they have been soaked in toxic creosote.
Timber size has a lot to do with the performance and lasting quality of a retaining wall. Timber size can vary widely. Common timber dimensions are, in inches, 4 by 4, 4 by 6, 5 by 6 and 6 by 6. The most common length is 8 feet. Five by 6 is the most common size. As with building a home, a retaining wall is only as good as the surface it sits on. The first row of timbers should be installed at the lowest point of the area being retained and then stepped up as necessary.
Some digging is required to embed about half of the lowest row of timbers. This practice prevents the pressure of the earth behind the wall from pushing it out.
To ensure that the lowest row of timbers is level, it might be necessary to lay some gravel into the trench and compact it with the flat head of a large sledge hammer. Once the lowest row is complete, additional layers can be added. Each row should be held back approximately an inch from the row below to offer greater strength and to prevent the wall from leaning. Joints also should be staggered a minimum of 2 feet.
As the wall grows higher, alternating rows should be overlapped at the corners and anchored with spikes consisting of metal rebar cut into 12-to-24 inch lengths and sharpened to a point at one end. Spikes should also be installed 8 inches from either end of each timber and at about 2-foot intervals along the timber. To facilitate installation of the spikes, a pilot hole should be drilled, using a bit 12 inches long and inch in diameter.
A traditional way of adding strength to a retaining wall is to cement support posts into the ground one half the height of the wall.
Unfortunately, this can add significantly to the cost and detract from the natural beauty of a timber retaining wall. The alternative is to install supports concealed in the soil behind the wall. The supports are called "deadmen." A deadman is built in a T shape with the long piece at a right angle to the wall and level to the ground. The length of the long piece is determined by the distance it must travel from the wall to the adjacent slope. The short piece should be about 2 feet long. Deadmen are tied to the wall and buried into the soil. They should be installed in a wall 3 or 4 courses high, or longer than 6 feet. The deadmen can be constructed mostly of scrap material from the retaining wall. Ideally, they should be located in the second or third row from the bottom and in the second row from the top. One rule of thumb is to install a deadman for every 16 square feet of retaining wall. A deadman will offer maximum benefit when located in undisturbed soil as opposed to fill which is more likely to give.
Once finished, pile gravel directly against the wall to improve drainage and minimize rot. Backfill the balance of the area with soil, using a landscape fabric between the gravel and the soil. The fabric will prevent the gravel from becoming compacted with soil that otherwise would impair drainage. Weep holes in the retaining wall or a perforated pipe (perforations down) surrounded by gravel at the base of the wall will relieve pressure on the wall and enhance drainage.